Within modern power systems, conventional synchronous generation (powered by fossil fuels) is increasingly being replaced by inverter-based renewable generation. As the penetration of renewable generation grows, there is an overall decrease in the inertia that exists within the power system. The regulation of frequency historically relied on high levels of inertia that existed within the network. Therefore, there is a growing need to supplement the missing inertia, ensuring the modern power system remains secure and stable. This thesis presents a controller for a modular multilevel converter (MMC) that quickly injects active power into an inertia deficient system and also supports various active power/load imbalances. Circulating currents in the MMC, which are traditionally eliminated, are injected into the arms of the MMC to manipulate the peaks of the arm currents. The maximum power controller flattens these peaks, allowing the MMC to be overloaded (by 27.5% in this work) without damaging its components. A linear relationship is established between the peak arm current (when the MMC is delivering 27.5% over its rated power) and the magnitude of the injected 2nd harmonic current (keeping the phase of the 2nd harmonic current constant). This property is used to develop the proportional-droop controller. The proposed method is validated in the PSCAD/EMTDC environment using both a simple test system and the IEEE 9-bus system. This thesis shows that circulating current can be used to manipulate the arm currents of the MMC such that MMCs can be used to deliver over-rated active power to the inertia deficient power systems.